A unique data base exists which may shed ligh on the relative importance of genetic versus environmental causes of schizophrenia. Studies over the last decade have proven that the nothwestern corner of Croatia has a high schizophrenia and manic depressive psychosis prevalence rate. Immigrants from this part of Croatia settled in northern Minnesota. Using Minnesota State Hospital records, it is proposed to ascertain the schizophrenia and manic depressive prevalence rates for both first and second generation Croatian immigrants. This will be compared with the rates for other immigrant groups in Minnesota and with native-born Americans. Since the town of origin of the immigrants is available through insurance records, the immigrants from the high prevalence Croatian area can be compared with those from the rest of Croatia. Furthermore a case register exists in Zagreb for all of Croatia, enabling contemporary comparisons to be made between immigrants and those who remained behind in specific areas of Croatia. The possible importance of age of immigration can also be examined. The research should yield important data for analysing various etiological theories of schizophrenia.